Virtual memory reclamation techniques
Virtual machines perform memory allocation in the same way, as an operating system handles memory allocation and deallocation. The guest operating system frees itself from a piece of physical memory by adding the memory page numbers to the guest free list.
The guest operating system free list is not accessible to the hypervisor and thus it is difficult for the hypervisor to know when to free host physical memory and when the guest physical memory is freed. The hypervisor is completely unaware of which pages are free or allocated in the guest operating system, and because of this the hypervisor cannot reclaim host physical memory when the guest operating system frees guest physical memory.
So VMware hypervisor relies on memory reclamation techniques to reclaim the host physical memory that is freed by the guest operating system. The memory reclamation techniques are:
Transparent page sharing
Memory ballooning
Host-level (or hypervisor) swapping
